The invention is concerned with ion implanters and with a method of ion implantation.
Ion implanters have been used for many years in the processing of semiconductor wafers. Typically, a beam of ions of a required species is produced and directed at a wafer or other semiconductor substrate, so that ions become implanted under the surface of the wafer. Implantation is typically used for producing regions in the semiconductor wafer of altered conductivity state, by implanting in the wafer ions of a required dopant. Typical ionic species used for this purpose are boron, phosphorous, arsenic and antimony. However, other ionic species are also used for other purposes, including oxygen for example.
The depth to which implanted ions penetrate the surface of the wafer is largely dependent on the energy of the ions in the ion beam. The semiconductor industry requires both very shallow implants, for example for very fine structures having a small feature size, and relatively deep implants, for example for buried layers etc. It is also a general requirement of the semiconductor processing industry that process times should be as short as possible which implies that the quantity of ions being implanted per unit area and time into a semiconductor wafer should be as high as possible. This implies that ion implantation is conducted with a high beam current, being a measure of the number of required ions in the beam reaching the wafer surface per unit time.
Beam energies up to about 200 keV (for singly charged ions) can quite readily be obtained using electrostatic acceleration systems, in which the source of ions is held at a fixed voltage relative to the wafer to be implanted, the fixed voltage defining the energy of the ions in the beam on implantation.
It has been recognized that radio frequency linear accelerators are useful to achieve higher beam energies.
A linear accelerator structure accelerates charged particles of a specific mass/charge ratio which are injected into the accelerator at a specific injection energy. It is the inherent nature of rf linear accelerators that the particles or bunches of particles passing through the accelerator must reach successive accelerating gaps at the right region of the sinusoidal waveform of the voltage applied to the gaps. Essentially, as each particle (or bunch of particles) crosses an accelerating cavity it will receive a certain amount of energy (increase in speed) dependent on the field across the gap at the specific time. If an accelerator is set up for particles of a particular mass/charge ratio and injection energy, the particles accelerated by a first gap will reach the next accelerating gap just as the field across that gap is optimum to provide further acceleration. It will be understood by people skilled in this art that a particle of the same energy but having a higher mass-to-charge ratio crossing the first gap would travel from the first gap at a lower velocity and so would tend to reach the next gap later in the rf wave form that is applied across that gap. Similarly, a lighter particle crossing the first gap would reach the second gap earlier. The accumulated effect of this over multiple accelerating gaps is that particles of mass-to-charge ratios different from the mass-to-charge ratio for which the accelerator is set up arrive at subsequent accelerating gaps at times when they are not suitably accelerated.
As is well known in the linear accelerator art to produce high energy beams of different ionic species the set up of the accelerator requires change to match the mass-to-charge ratios of the selected ions. Among ions useful for implantation, singly charged boron (B+) has a mass/charge ratio of about 11, whereas singly charged phosphorous (P+) has a mass/charge ratio of about 31. Singly charged arsenic has a mass-to-charge ratio of about 75 and singly charged antimony has a mass-to-charge ratio of about 122.
The use of rf linear accelerators for ion implantation has been suggested at least since 1976 in xe2x80x9cUpgrading of Single Stage Acceleratorsxe2x80x9d by K. Bethge et al, pages 461-468, Proceedings of the Fourth Conference on the Scientific and Industrial Applications of Small Accelerators, North Texas State University, Oct. 27-29, 1976; and in xe2x80x9cHeavy Ion Post-acceleration on the Heidelberg MP Tandem Acceleratorxe2x80x9d, edited by J. P. Wurm, Max Planck Institute for Nuclear Physics, Heidelberg, May 1976. U.S. Pat. No. 4,667,111 discloses an ion implanter incorporating a radio frequency linear accelerator to provide ultimate beam energies as high as 2 meV or more. The rf linear accelerator is formed of a series of so called two gap accelerating cavities. For set up of the accelerator, with the frequency of the rf fields in successive cavities of the accelerator kept the same, the phase of the wave form for one two-gap cavity relative to the preceding two-gap cavity is adjusted so that the correct point of its waveform is presented to arriving ions of the selected species. The resulting two-gap tool tends to be very long relative to the performance achieved; the specification contemplates using ten or more two-gap cavities in succession, and is limited to relatively low beam currents. Whereas a low beam current may be satisfactory at high energies, when the apparatus is operating at relatively lower energies, higher beam currents are desirable to improve the processing speed.
Japanese Patent Application Publication No. Hei 9-237700 (1997) discloses an ion implanter using an rf accelerator formed with one or more three gap rf accelerator cavities. In this context it will be understood by those skilled in the art of linear accelerators that a two gap accelerator cavity, e.g. as used by the above referred U.S. patent, has entrance and exit electrodes at a fixed, usually ground, potential and a single intermediate electrode to which is applied the rf potential, thereby forming a pair of accelerating gaps on opposite sides of the rf electrode. As is also well known in the art, a three gap cavity has entrance and exit electrodes at a fixed, usually ground, potential and a pair of intermediate electrodes defining three gaps. The intermediate electrodes are energised by the rf potential with opposite polarity. Thus, if the amplitude of the energising rf voltage is V, the maximum accelerating potential across the first and last gaps of the cavity is V whereas the maximum accelerating potential between the two intermediate electrodes is 2V.
In the above Japanese publication, the injection energy to the three gap rf accelerator cavity appears to be relatively high. The specification contemplates some form of beam accelerator upstream of the three gap cavity but downstream of the usual analyser magnet, which separates from the ions emitted from an ion source the particular species of ion required for implantation. U.S. Pat. No. 5,801,488, which is assigned to the same Assignee as the above Japanese patent publication, discloses the provision of an rf quadrupole accelerator upstream of the three-gap linear accelerator stages.
Reference may also be made to Japanese Patent Publications Nos. Hei 7-57897 and Hei 7-57898 which disclose features of the same machine, and also to the article xe2x80x9cThe development of a beamline using an RFQ and three gap rf accelerators for high energy ion implanterxe2x80x9d, Fujisawa et el, presented at IIT, Kyoto, Jun. 24th 1998.
Generally, the above Japanese references disclose an implanter tool which is likely to be very large and expensive to build. Furthermore, beam currents when operating at relatively lower energies will be very small.
An object of the present invention is to provide an ion implanter using at least one rf accelerator stage, which can generate a high energy beam as well as operate at lower energies with good beam current.
Accordingly, in one aspect the invention provides an ion implanter comprising an ion beam generator for generating a beam of ions to be implanted in which said ions are at a first energy, and a radio frequency linear accelerator assembly arranged when energised for accelerating ions of said beam to a second energy, said assembly comprising electrodes defining a series of gaps for changing the energy of ions of said beam, said electrodes having apertures through which the ions pass, wherein the apertures of the electrodes defining the gaps of the accelerator assembly have respective first dimensions in a first orthogonal direction transverse to the beam direction and respective second dimensions in a second orthogonal direction transverse to the beam direction, said first dimension of the aperture of at least the first electrode defining the first gap being smaller than said second dimension of said first electrode aperture. The aperture of this first electrode may be slit shaped. With such an electrode aperture shape the smaller first dimension is effective to limit field penetration into the aperture. This is important to ensure the potential within the electrode aperture, even at the position of the central axis of the beam, is closely similar to the potential of the electrode. Excessive field penetration might otherwise necessitate making the electrode longer in the beam direction which would require a lower frequency rf voltage. Also, reducing field penetration can improve efficiency by increasing the so called xe2x80x9ctransit time factorxe2x80x9d of the gaps of the accelerator. On the other hand, the larger second dimension can reduce the focusing effect of the field in said second orthogonal direction for ions passing through the electrode aperture in a beam having a width in said second orthogonal direction which is significantly less than said second dimension.
Apart from the first electrode of the assembly in the beam direction, the second electrode may also have an aperture with its first dimension smaller than its second dimension. Preferably, all the electrodes of at least a first cavity of the assembly will be so formed.
This structure, especially in combination with magnetic quadrupoles for beam focusing, will allow higher beam current through the accelerator assembly, not only when energised for accelerating beam ions, but also when operating in drift mode. In this context xe2x80x9cdrift modexe2x80x9d denotes operating the ion implanter with no rf voltage applied to any of the electrodes of the rf accelerator assembly, so that ions are implanted at the xe2x80x9cfirst energyxe2x80x9d or the energy of injection into the rf accelerator, or even lower energies if a deceleration system is provided.
In order to minimise the overall length of the linear accelerator assembly the assembly is preferably formed of at least one three gap linear accelerator stage. A linear accelerator formed of three gap cavities can be shorter overall for the same energy increment as will be provided by an equivalent accelerator formed of two gap cavities.
The invention also provides an ion implanter comprising an ion beam generator for generating a beam of ions to be implanted, in which said ions have a predetermined mass/charge ratio and an injection energy E, a three gap linear accelerator stage into which said beam of ions is directed at said injection energy, said stage being arranged when energised for accelerating ions of said beam to a second energy, said stage comprising an entrance electrode held at a fixed potential and an exit electrode held at a fixed potential, first and second radio frequency electrodes located in series between said entrance and exit electrodes, and a radio frequency generator to apply radio frequency voltages of opposite polarity and a predetermined frequency f respectively to said first and second electrodes, said entrance electrode and said first radio frequency electrode defining a first accelerating gap, said first and second radio frequency electrodes defining a second accelerating gap having a centre point at a first predetermined spacing d1 from the centre point of the first gap, and said second radio frequency electrode and said exit electrode defining a third accelerating gap having a centre point at a second predetermined spacing d2 from said centre point of the second gap, wherein the injection energy E, the frequency f, and the gap spacings d1 and d2, are selected such that, at amplitudes of the radio frequency energy below the maximum amplitude at which breakdown occurs across any of said gaps, injected ions of said beam which cross the first gap when the radio frequency field across the first gap is rising from a maximum deceleration field to a maximum acceleration field, then cross the second gap during the maximum acceleration field across the second gap and cross the third gap when the field across the third gap is falling from a maximum acceleration field to a maximum deceleration field.
According to classical practice in the operation of linear accelerators, the accelerator should be driven and structured so that bunches of ions passing along the length of the accelerator arrive at each accelerating gap at or shortly before the peak of the rf field across that gap which would produce maximum acceleration. By arriving shortly before the peak, the variation in field strength experienced by ions in the bunch arriving at different times tends to provide greater acceleration to ions arriving late in the bunch, and less acceleration to ions arriving first in the bunch. Thus, the bunching tendency is maintained as the ions pass through the accelerator.
It has been found, however, that there are significant advantages in operating a three gap accelerator stage so that ions arriving at the first gap before the point of maximum acceleration field across the first gap, reach the second gap during maximum acceleration field and cross the third gap after the point of maximum acceleration field. Setting up the accelerator stage in this way maximises the acceptance of ions from the beam injected into the accelerator. Energy spread introduced to the ions crossing the first gap tends to be removed again as the ions cross the third gap. As a result, the accelerator stage can accept ions crossing the first gap over a greater spread of rf phase angles, for a desired percentage spread in the energies of ions leaving the stage. If the spread of energies introduced by the first gap is effective to reduce the spread in phase of the ions by the time they reach the third gap, the third gap may not so effectively remove the energy spread. However, such ions would also have reduced spread in phase when crossing the second gap,so that the energy spread introduced by the second gap would be reduced. The overall effect would be a similar reduction in overall percentage energy spread in the ions leaving the accelerator stage.
Importantly, also, such an arrangement allows the stage to be used with different applied radio frequency voltage amplitudes. Reducing the rf amplitude from the maximum has the effect of reducing the energy increment delivered to the beam ions passing through the stage. Importantly, by constructing the accelerator stage to operate in the way described above, the proportion of injected ions passing through the accelerator stage and accelerated to the target energy is maintained over a wide range of applied rf voltages. This allows the accelerator stage to be operated for delivering a range of energy increments to the injected ion beam while still maintaining good beam current in the accelerated beam.
This is especially important for ion implantation where it is crucial that ions are delivered to the target substrate at a reasonably well defined energy. Excessive energy spread in ions accelerated by an rf linear accelerator stage would result in fewer ions in the beam having the required target energy, thereby reducing the effective beam current at the required energy on the target substrate.
The invention still further provides a method of implanting ions into a target substrate comprising the steps of generating a beam of the ions at a first energy, and changing the energy of ions in the beam to a second energy using a radio frequency (rf) linear accelerator assembly having at least first and second booster stages in tandem along the beam direction, each of the booster stages comprising entrance and exit electrodes and at least one intermediate rf electrode defining a series of gaps for changing the energy of ions of said beam, the exit electrode of the first booster stage and the entrance electrode of the second booster stage defining between them a drift distance between the stages over which beam ions are not subject to rf fields, wherein the speed of the bunches of ions from the first booster stage over said drift distance to the second booster stage, and thus the flight time, is adjusted, while locking the phase of the field at each stage to a respective fixed value. This provides a very simple and convenient method of controlling the arrival time of the bunches of ions at the second stage of the linear accelerator. As a result, rf acceleration is used for accelerating ion species over a range of mass to charge ratios to energies useful for high energy ion implantation without having to resort to the complication of independently varying the phases of the two booster stages.
One way of achieving this is by adjusting the amplitude of the rf fields in the first booster stage, thereby adjusting the energy, i.e. speed, of the bunches exiting the first booster.
In particular, the set up of the linear accelerator may be changed from accelerating a beam of ions of a first mass/charge ratio to accelerating a beam of ions of a second mass/charge ratio by changing ion speed over the drift distance while maintaining the respective phases of the rf fields in the first and second booster stages locked. In this way the set up of the accelerator can easily be changed for accelerating ions of different mass charge ratios.
Preferably, the drift distance is greater than the length of the first booster stage between the entrance and exit electrodes thereof. Then, a relatively modest change in the speed (energy) of the bunches of ions exiting the first booster stage can have a substantial effect on the time of arrival of the bunches at the second booster stage.
In a further aspect, the present invention provides an ion implanter comprising an ion beam generator for generating a beam of ions to be implanted, in which said ions are at a first energy, and a radio frequency (rf) linear accelerator assembly arranged, when energised, for accelerating ions of said beam to a second energy, said assembly comprising at least first and second resonant cavities in tandem along the beam direction, said cavities comprising electrodes defining a series of gaps for changing the energy of ions of said beam, a rf power supply to provide a first supply of rf energy at a first frequency to said first cavity, said first cavity being resonant at said first frequency, whereby to produce corresponding first rf accelerating fields between electrode gaps in said first cavity, said first fields having a phase and an amplitude, said rf power supply providing a second supply of rf energy at a second frequency, which is the same as or a harmonic of said first frequency, to said second cavity, said second cavity being resonant at said second frequency, whereby to produce corresponding second rf accelerating fields between electrode gaps in said second cavity, said second fields having a phase and an amplitude, and a controller arranged to adjust the time of flight of bunches of ions from the first cavity to the- second cavity by adjusting the amplitude of said first accelerating fields in said first cavity, while maintaining locked to fixed values the phases of said first and second fields.
There follows by way of example only a description of a preferred embodiment of the invention.